Place the potatoes in a large saucepan and cover them with cold, generously salted water.
Bring the water to a boil, then reduce the heat slightly and cook the potatoes for 12 to 15 minutes, or until they are tender when pierced with a knife but still hold their shape.
Drain the potatoes well and leave them to cool for about 5 minutes.
While the potatoes are cooking, preheat a grill, grill pan or cast-iron skillet over medium-high heat.
Brush the corn lightly with olive oil and season it with a small pinch of salt and black pepper.
Grill the corn for 8 to 10 minutes, turning regularly, until the kernels are tender and lightly charred in several places. Transfer it to a cutting board and allow it to cool slightly.
Prepare the dressing by whisking together the mayonnaise, Greek yogurt, lemon juice, lemon zest, Dijon mustard, garlic, sugar or honey, salt and black pepper.
Place the warm potatoes in a large salad bowl. Add approximately half of the dressing and gently toss until the potatoes are coated. Leave them to cool for another 10 minutes.
Stand each ear of corn upright on the cutting board and carefully slice downward to remove the kernels.
Add the grilled corn, tomatoes, red onion, parsley and chives to the potatoes.
Pour over the remaining dressing and gently toss everything together. Taste and adjust the seasoning with additional salt, pepper or lemon juice if needed.
Serve immediately while slightly warm, or refrigerate the salad for 30 minutes if you prefer it chilled.